MOLLUSCS. 



253 



istic features of these forms, — a shell divided into halves, one on either side of the 

 body. This bilateral symmetry pervades the wliole organism, and frecjuently one 

 side is almost an exact repetition of the other. Just inside the shell is found the 

 fleshy mantle, which, like the shell it secretes, forms a flap on either side of the body. 

 In these bivalves this pallium, or mantle, acquires a great development, and not infre- 

 (juently its edges are joined together, so that the rest of the animal is enveloped, as it 

 were, in a bag. Still the bag is never completely closed ; at tlie front end a small 



Fig. 2G0. — Diagram of anatomy of a clam {Mt/a); a, anterior 



siphonal tube; e, incurrent sipUoiial tube; /".foot; ,'/, gills; (, nit. 

 retractor; ;-, retractor of foot; t, labial palpi; v, ventricle uf lie-ari. 



hole is left for the ])rotrusion of the foot, while, at the op|)osite exti'emity, means is 

 afforded for tlie entrance of water, bringing food and o.vygen to tlie animal, and also 

 for the escape of the .same fluid, bearing away the waste products of respiration and 

 digestion. Not infrequently this posterior opening becomes divided into two tubes, 

 which sometimes can be extended a long distance from the shell. This is known as 

 the siphon, and will readily be recognized by most people in the ' head ' of the clam. 

 He.ad it certainly is not, for it is at exactly the opposite end of the body from where 

 the head should be. These tubes, which are, in reality, but exj>ansions of the mantle, 

 are very contractile, and each tube has its own function. The lower one (the one fur- 

 thest from the hinge of the shell) is for the incurrent stream, while from the otlier 

 the water which has played its part in the economy of the animal is discharged. In 

 other forms there is no siphon, and in still others 

 the two halves of the mantle are entirely free 

 from each other. 



The mantle joins the body near the hinge 

 line, and between the two hang down the gills, 

 to which we sliall again recur. From the lower 

 side of the body proceeds the foot, which in some 

 forms is well developed, even proving an organ 

 of locomotion of no mean capacities, while in 

 others, like the oyster, the foot has nearly, or even 

 entirely, disa])pcared. Xear the extremity of the foot in the adults of some species, 

 and the young of others, is a gland, the function of which is to secrete the byssus. 

 This is a bundle of fibres, more or less closely united, by which the animal attaches 

 itself. The byssus can be cast off by the animal when desired, and a new one formed 

 at pleasure. 



The mouth, which is at the opjjosite end of the body from tli;it at wliicli tln' siplidii 



Fig. 2(;i. — Mussel with byssal tliread.'* 



